A 6.8 magnitude earthquake struck southern Alaska early Sunday, US seismologists said.

The US Geological Survey, updating its earlier location for the quake, said it struck at 1030 GMT and was centered about 83 kilometers (51 miles) east of Old Iliamna.

The USGS, which initially had assessed the magnitude of the quake as 7.1, estimated that it had a depth of 127 kilometers (79 miles).

There is only a low likelihood of casualties and damage, the USGS said, adding that "weak" shaking from the quake could be felt in nearby cities.

One of the cities where it could be felt was Anchorage, the closest large city, some 162 miles (259 kilometers) from the epicenter.

Twitter was awash with posts -- including from Anchorage -- saying the shaking was pronounced and prolonged.

"Everyone in Anchorage is awake and on Twitter right? Biggest longest #earthquake of my entire life. Family is all hanging in our bed now," wrote @jgrenn.

Another Twitter user, @Shelbae97, wrote: "Thank Alaska for that earthquake," and posted a photo a broken mirror and other household objects flung to her feet.

A local power company, Matanuska Electric Association, said in a statement that "the 7.1m earthquake at 1:30 am caused several outages," including to two power substations.

A local television station, KTUU, reported that about 4,900 Matanuska customers were without power.

"Crews have been called in from home to assess the situation and restore power," the power company said.

The Hawaii-based Pacific Tsunami Warning Center said there was no risk of a tidal wave "because the earthquake is located too deep inside the earth."